1. Field of the Invention
The present invention relates to a light exposure system suitable for production of semiconductors, and more particularly, to a light exposure system provided with an oblique illumination optical system illuminating in an oblique direction a projection original plate on which a circuit pattern or the like is formed.
2. Related Background Art
First prior art
Semiconductor integrated circuits have been increasing the degree of integration these days so that a minimum line width in a circuit comes to be of submicron order. With the increase of integration degree, projection exposure systems for producing semiconductor integrated circuits in the photolithography process are now required to have a higher resolution and a higher alignment accuracy. Positioning or alignment includes wafer alignment for aligning a position to be exposed for each shot on a wafer with a position where an image of a pattern of a reticle is projected to form an image thereto on the wafer, and reticle alignment for maintaining a positional relation in a certain state between the reticle and a stage upon exchange of reticle.
FIG. 6 shows a projection exposure system provided with a measurement optical system for performing the conventional reticle alignment. In FIG. 6, reference numeral 1 designates a light source such as a mercury lamp. Exposure light IL radiated from the light source 1 is condensed by an elliptic mirror 2 and then reflected by a bending mirror 3. Then, the light IL passes through an input lens 4 to become a parallel beam. The parallel beam enters an optical integrator 5 composed of a fly eye lens. A shutter 6 is located at the vicinity of the secondary focus of the elliptic mirror 2. The exposure light IL may be intercepted on demand by rotating the shutter 6 through a drive motor 7.
When the exposure light IL is intercepted by the shutter 6, the exposure light IL is reflected by the shutter 6 in a direction substantially normal to the optical axis of the elliptic mirror 2. The thus-reflected exposure light IL is made by a condenser lens 8 to enter an end of a light guide 9. The exposure light IL radiated from the light source IL enters either the optical integrator 5 or the light guide 9, accordingly.
When the exposure light IL enters the optical integrator 5, numerous secondary light source images, as will be hereinafter referred to as "secondary light source", are formed on a focal plane of the optical integrator 5 on the reticle side. A variable aperture stop 10 is positioned at the secondary light source formation plane. The exposure light emitted from the secondary light source passes through a half mirror 11 inclined at 45 degrees to the optical axis of the exposure light. Then the exposure light passes through a first condenser lens 12, a bending mirror 13, and a second condenser lens 14 then to illuminate at a uniform illuminance a pattern area on the lower surface of a reticle R.
In FIG. 6, PL represents a double side (or one side) telecentric projection optical system and 17 does a wafer stage. A wafer W is placed in a wafer holder on the wafer stage 17. The wafer stage 17 is comprised of an X-Y stage, which can two-dimensionally move in a plane normal to the optical axis of the projection optical system PL, and a Z stage, which can move in the direction of the optical axis of the projection optical system PL. An image of the pattern on the reticle R is formed by the projection optical system PL on the wafer W during exposure. In this arrangement, the secondary light source formation plane of the optical integrator 5 is conjugate with a pupil plane of the projection optical system PL. The .sigma. value, which represents the coherency of the illumination optical system illuminating the reticle R, may be changed by adjusting an aperture of the variable aperture stop 10 located at the secondary light source formation plane. Defining the maximum incident angle of the exposure light IL illuminating the reticle R as .theta..sub.IL and a half angular aperture of the projection optical system PL on the side of the reticle R as .theta..sub.PL, the .sigma. value may be expressed by sin .theta..sub.IL / sin .theta..sub.PL. The .sigma. value is set at about 0.3 to 0.6 in general.
An adjustment plate 19, which is a glass plate for example, is fixed near the wafer holder on the wafer stage 17. A reference pattern is formed on a plane of the adjustment plate 19 on the side of the projection optical system PL. Corresponding to the reference pattern, a reticle mark RM is formed in an image field of the projection optical system and in the proximity of the pattern area on the reticle R at a position where the reticle mark is conjugate with the reference pattern on the adjustment plate 19 with respect to the projection optical system PL. For example, the reference pattern on the adjustment plate 19 may be a cross aperture pattern formed in a light shielding surface, while the reticle mark RM on the wafer W may be a pattern obtained by multiplying the reference pattern by a projection magnification of the projection optical system PL and then reversing bright and dark portions in the pattern.
There are a condenser lens 21 and a bending mirror 20 placed below the adjustment plate 19 of the wafer stage 17. The other end of the light guide 9 is fixed at the rear focal plane of the condenser lens 21. A surface of the other end of the light guide 9 is conjugate with a pupil plane of the projection optical system PL, and, therefore, with the variable aperture stop 10. A light emission plane of the other end of the light guide 9 is so sized as to provide a projection image onto the variable aperture stop 10, which is substantially equal to the aperture of the variable aperture stop 10. In this arrangement, the emission plane illuminates the reference pattern on the adjustment plate 19 with an illumination .sigma. value almost equal to that for exposure light IL. Further, in the illumination optical system for exposure light IL, a light receiving portion of a photomultiplier 22 is located at a position conjugate with the variable aperture stop 10 with respect to the half mirror 11. Namely, the light receiving portion of the photomultiplier 22 is positioned in a plane conjugate with the pupil plane of the projection optical system PL and with the emission plane of the other end of the light guide 9. A detection surface in the light receiving portion is larger than an image of the light emission plane of the other end of the light guide 9 projected thereon, whereby preventing a loss in quantity of light. Consequently, when the reference pattern of the adjustment plate 19 is illuminated from the bottom side, most of light emitted through the reference pattern of the adjustment plate 19 enters the projection optical system PL and then reaches through the reticle mark RM on the reticle R to the light receiving surface of the photomultiplier 22 no matter where the adjustment plate 19 is situated in the image field of the projection optical system PL.
In FIG. 6, numeral 23 denotes a central processing unit (CPU) for controlling an operation for reticle alignment. Photoelectrical conversion signals are output from the photomultiplier 22 to the CPU 23. There are an X direction mirror and an unrepresented Y direction mirror fixed on the upper surface of the wafer stage 17. A coordinate of a position may be always monitored on the wafer stage 17 by using a laser interferometer 24 and the two mirrors. Coordinate information of the wafer stage 17 is supplied from the laser interferometer 24 to the CPU 23, and the CPU 23 may move the wafer stage 17 through the stage driving unit 25 to a desired coordinate position.
In reticle alignment, the CPU 23 makes the shutter 6 intercept the exposure light IL through the drive motor 7 and makes the adjustment plate 19 on the wafer stage 17 move into the image field of the projection optical system PL through the stage driving unit 25. By this, the exposure light IL reflected by the shutter 6 passes through the condenser lens 8 and the light guide 9 to be emitted into the inside of the wafer stage 17. The exposure light thus emitted will be hereinafter referred to as illumination light. The illumination light is reflected by the bending mirror 20 and thereafter converted by the condenser lens 21 into a parallel beam to illuminate the reference pattern formed on the adjustment plate 19 from the bottom side thereof. A zeroth order beam directly transmitted through the reference pattern of the adjustment plate 19 and illumination light diffracted by the reference pattern are converged by the projection optical system PL at the position conjugate with the reference pattern on the reticle R to form projection images of the reference pattern there.
Out of the light beams forming the projection images of the reference pattern, beams of the illumination light having passed through a light transmissive area of the reticle mark RM on the reticle R pass through the second condenser lens 14, the bending mirror 13, the first condenser lens 12, and the half mirror 13 to enter the light receiving surface of the photomultiplier 22. The higher order diffracted light beams, out of the beams of the illumination light output from the reference pattern of the adjustment plate 19 toward the projection optical system PL, are slant at large angles to the optical axis of the second condenser lens 14 when passing through the reticle mark RM of the reticle R. Consequently, most of the higher order diffracted light beams from the adjustment plate 19 do not reach the light receiving surface of the photomultiplier 22. However, the light intensity of the higher order diffracted light is very low. Therefore, as far as the adjustment plate 19 is present in the image field of the projection optical system PL, most of the illumination light from the adjustment plate 19 enters the light receiving surface of the photomultiplier, whereby the SN ratio of the photoelectric conversion signals is high enough.
The CPU 23 controls the stage driving unit 25 to scan the wafer stage 17 by the laser interferometer 24 in a plane normal to the optical axis of the projection optical system PL while monitoring coordinate information thereof. When the projection image of the reference pattern of the adjustment plate 19 on the wafer stage 17 is superimposed on the reticle mark RM, which has the pattern with inverted bright and dark portions to the reference pattern, the minimum output is obtained in a photoelectric conversion signal of the photomultiplier 22 supplied to the CPU 23. Therefore, a coordinate of the stage where the minimum output is obtained in the photoelectric conversion signal corresponds to the position where the projection image of the reference pattern of the adjustment plate 19 is superimposed on the reticle mark RM on the bottom surface of the reticle R. Then, measuring stage coordinate values of crosses RM disposed at at least two positions near the pattern area of the reticle R, a coordinate of the center position and a rotation angle of the projection image of the reticle R projected onto the upper surface of the wafer stage 17 may be measured at high accuracy. For example, the reticle R is finely moved by the reticle driving mechanism (not shown) under control of the CPU 23 in the plane normal to the optical axis of the projection optical system PL, and the center position and the rotation angle in the horizontal plane of the projection image of the reticle R are set on the upper surface of the wafer stage 17 in desired conditions, whereby effecting the alignment of the reticle R.
Next explained with reference to FIG. 7A and FIG. 7B is the fundamental structure of the conventional illumination optical system for exposure light IL. FIG. 7A simplifies the main part of the illumination optical system of the projection exposure system as shown in FIG. 6. In FIG. 7A reference numeral 26 designates a condenser lens, and the exposure light passes through the optical integrator 5 comprised of the fly eye lens, the variable aperture stop 10, and the condenser lens 26 to illuminate the pattern 15a on the reticle R. The variable aperture stop 10 is located at the reticle side focal plane P1 of the optical integrator 5, that is, at or near the Fourier transform plane to the pattern 15a of the reticle, and has an aperture 10a of substantial circle with the center at the optical axis AX of the projection optical system PL, as shown in FIG. 7B. The variable aperture stop 10 defines a circular circumference of the numerous images in the secondary light source formed on the reticle side focal plane P1.
A part of the exposure light with a wavelength .lambda. illuminating the reticle R is diffracted by the pattern 15a of the reticle R, and a zeroth order light beam D.sub.o, a positive (+) first order diffracted light beam D.sub.p, and a negative (-) first order diffracted light beam D.sub.m are incident from the pattern 15a into the projection optical system PL. These zeroth order light beam and .+-. first order diffracted light beams are condensed by the projection optical system PL onto the wafer W to form interference fringes on the wafer W. In this case, defining the pupil plane of the projection optical system PL as P2, the higher order diffracted light beams become apart from the optical axis on the pupil plane P2 with an increase of the order. If at least .+-. first order light beams could pass through the pupil plane P2, a pattern would be formed on the wafer W at a pitch obtained by multiplying a pitch P of the pattern 15a of the reticle R by the projection magnification. When a numerical aperture of the projection optical system PL on the side of the reticle R is defined as NA.sub.R, the minimum resolution limit of the pattern 15a of the reticle R, which can be resolved by the projection optical system PL, is .lambda./NA.sub.R.
The minimum pattern size is a half of the minimum pitch P, that is, 0.5.multidot..lambda./NA.sub.R. In an actual application, influences should be considered for example from the wafer warp and the photo resist in photolithography process. A practical minimum pattern size on the reticle R is k.multidot..lambda./NA.sub.R, where k is a process factor of about 0.6 to 0.8. Further, when the numerical aperture of the projection optical system PL on the side of the wafer W is defined as NA.sub.W, a depth of focus of the projection optical system PL is represented by .+-..lambda./{2(NA.sub.W).sup.2 }, and there stands such a relation that NA.sub.W =NA.sub.R /.beta., where .beta. is an imaging magnification (reduction ratio) of the projection optical system PL. Accordingly, as the numerical aperture of the projection optical system PL is increased in order to decrease the minimum pattern size on the reticle, the depth of focus becomes shallower in proportion to the inverse square of the numerical aperture.
It is a recent need that a reticle pattern with a further smaller pattern size be replicated on a wafer in order to obtain a semiconductor integrated circuit further highly integrated. There is, however, a disadvantage of a too shallow depth of focus resulted, if the minimum pattern size is reduced by increasing the numerical aperture of the projection optical system 16 in the illumination method in which the reticle R is illuminated through the condenser lens 26 with the exposure light from the secondary light source formed by the optical integrator 5 disposed on the optical axis as shown in FIG. 7A.
There is so-called plural oblique illumination proposed as shown in FIG. 8 as an illumination method for high resolution, by which the minimum pattern size may be made smaller with an acceptable reduction of the depth of focus. In FIG. 8, reference numerals 29a and 29b represent a first group of fly eye lenses (first optical integrators), and 27a and 27b a second group of fly eye lenses (second optical integrators) placed along an extension of the first group of fly eye lenses so that the second group of fly eye lenses 27a and 27b are illuminated by the exposure light emitted from the light source IL.
The exposure light beams output from the second group of fly eye lenses 27a and 27b are condensed by the guide optical systems 28a and 28b, respectively, then to uniformly illuminate the first group of fly eye lenses 29a and 29b. Variable aperture stops 30a and 30b are positioned on the reticle side focal plane P1 of the first group of fly eye lenses 29a and 29b, respectively. The variable aperture stops 30a and 30b define outer diameters of secondary light sources formed on the reticle side focal plane P1 of the first group of fly eye lenses 29a and 29b. The exposure light output from the variable aperture stops 30a and 30b uniformly illuminates the pattern 15a of the reticle R through the condenser lens 26. Since the first and second groups of fly eye lenses make the illuminance distribution uniform, uniformity of the illuminance distribution of the exposure light becomes remarkably excellent on the reticle R.
In this arrangement, the variable aperture stops 30a and 30b are disposed on the object side focal plane of the condenser lens 26, the reticle R is located on the image side focal plane of the condenser lens 26, and the secondary light source formation plane P1 is conjugate with the pupil plane P2 of the projection optical system PL. The exposure light output from the variable aperture stop 30a is refracted by the condenser lens 26 such that a ray leaving the center of the variable aperture stop (as will be referred to as "principal ray") enters the reticle R as inclined at an angle .phi.(0&lt;.phi.&lt;.pi./2) in the clockwise direction with respect to the optical axis AX of the projection optical system PL. In contrast, the exposure light output from the variable aperture stop 30b is refracted by the condenser lens 26 such that a principal ray enters the reticle R as inclined at an angle .phi. in the counterclockwise direction with respect to the optical axis AX of the projection optical system PL.
Supposing the reticle pattern 15a has a grating pattern formed at a predetermined pitch P in a direction parallel to the sheet plane of FIG. 8 and when the exposure light output from the variable aperture stop 30a is incident into the reticle pattern 15a, the zeroth order beam D.sub.o goes out thereof toward the projection optical system PL in the same direction as the incident light. The + first order diffracted light beam D.sub.p goes out in a direction which is inclined at an angle of (.theta..sub.p +.phi.) in the clockwise direction to the optical axis AX, while the--first order diffracted light beam D.sub.m goes out in a direction which is inclined at an angle of (.theta..sub.m -.phi.) in the counterclockwise direction to the optical axis AX. These diffraction angles .theta..sub.p and .theta..sub.m may be expressed by the following equations. EQU sin (.theta..sub.p +.phi.)-sin .phi.=.lambda./P (1) EQU sin (.theta..sub.m -.phi.)+sin .phi.=.lambda./P (2)
As the reticle pattern 15a becomes finer, the + first order diffracted light beam D.sub.p, which is output in the direction inclined by the angle (.theta..sub.p +.phi.), comes to fail to pass through the aperture stop on the pupil plane P2 of the projection optical system PL. In such a case, however, the--first order diffracted light beam D.sub.m may pass through the aperture stop of the projection optical system PL. Then, there are interference fringes made by the two beams of the zeroth order light beam D.sub.o and the--first order diffracted light beam D.sub.m on the wafer W. The interference fringes show an image of the reticle pattern 15a. If the reticle pattern 15a has a line and space pattern of 1:1, an image of the reticle pattern 15a is formed at a contrast of about 90% on a resist coating on the wafer W.
In this case, a resolution limit is set when following condition is established. EQU sin (.theta..sub.m -.phi.)=NA.sub.R ( 3)
Putting the equation (3) into the equation (2), the following relation stands with the pitch P of the minimum pattern which can be replicated, on the reticle side. EQU NA.sub.R +sin .phi.=.lambda./P
Arranging the above relation, the pitch P of the minimum pattern is obtained as follows. EQU P=.lambda./(NA.sub.R +sin .phi.) (4)
For example, if the numerical aperture, sin .phi., for the inclination angle .phi. of the exposure light illuminating the reticle R is set to 0.5.times.NA.sub.R, the pitch P of the minimum pattern is obtained as follows. EQU P=.lambda./(NA.sub.R +0.5NA.sub.R)=2.lambda./(3NA.sub.R) (5)
Comparing to the resolution limit of .lambda./NA.sub.R in the illumination method of FIGS. 7A and 7B, the illumination method of FIG. 8 can provide a higher resolution as shown.
The following discussion is about the depth of focus. If a deviation amount (defocus amount) of the wafer W from the focus position of the projection optical system PL is .DELTA.F and an incidence angle at which the zeroth order beam or a diffracted beam is incident at a point on the wafer W is .theta..sub.D, the wavefront aberration due to the defocus is given by .DELTA.F.multidot.sin.sup.2 .theta..sub.D /2. In this aberration, sin .theta..sub.D represents a distance of the diffracted light to the optical axis AX on the pupil plane P2 of the projection optical system PL. In the conventional illumination method as shown in FIGS. 7A and 7B, sin .theta..sub.D =0 for the zeroth order light beam D.sub.o. On the other hand, sin .theta..sub.D =M.multidot..lambda./P for the .+-. first order diffracted light beams D.sub.p, D.sub.m, where M is the magnification (enlargement ratio) of the projection optical system. Therefore, the wavefront aberration due to the defocus with the zeroth order light D.sub.o and the .+-. first order diffracted light beams D.sub.p, D.sub.m is .DELTA.F.multidot.M.sup.2 (.lambda./P).sup.2 /2.
In contrast, sin .theta..sub.D =M.multidot.sin .phi. in the illumination method of FIG. 8, since the zeroth order beam D.sub.o is inclined at the angle .phi. to the optical axis AX. As for the--first order diffracted light beam D.sub.m, sin .theta..sub.D =M.multidot.sin (.theta..sub.m -.phi.). In this case, if the diffraction angle .theta..sub.m is about 2.phi., sin .phi..perspectiveto.sin (.theta..sub.m -.phi.), whereby the relative wavefront aberration due to the defocus becomes almost equal to 0 between the zeroth order light beam D.sub.o and the--first order diffracted light beam D.sub.m. Then, even if the wafer W is slightly deviated in the direction of the optical axis from the focus position, the image of the reticle pattern 15a is not affected so much as to the focus condition thereof as compared to the conventional method, resulting in enlarging the depth of focus. In order to set the diffraction angle .theta..sub.m to 2.phi., the incident angle .phi. of the exposure light into the reticle R should be set to satisfy the relation sin .phi.=.lambda./(2P), as seen from the equation (2). By this, the depth of focus may be greatly increased.
Problems in the first prior art
In the plural oblique illumination as shown in FIG. 8, the .sigma. value is for example about 0.2 for the exposure light output from each of the fly eye lenses 29a and 29b, but the process is substantially equivalent to what the condenser lens 26 processes the exposure light from a light source having a circular circumference circumscribing the fly eye lenses 29a and 29b. Thus, the condenser lens 26 has to illuminate the reticle R with an illumination beam having a large .sigma. value for example of about 1, and the requirements accuracy thereof has been increasing higher and higher these days. In detail, the condenser lens 26 has to be designed to have improved illuminance uniformity and illumination telecentricity by using a beam having a large .sigma. value, which forces a great load on the design of such a condenser lens.
In addition, in order to provide the exposure system with the measurement optical system for reticle alignment, the half mirror 11 must be located between the condenser lens, which is the condenser lens 12 or 14 in the example of FIG. 6, and the optical integrator (or the variable aperture stop), as shown in FIG. 6. However, if a beam for reticle position measurement is taken out by placing a half mirror in an optical axis between the condenser lens 26 and the optical integrator or the variable aperture stop, as being the case in the conventional method, in the illumination method as shown in FIG. 8, the condenser lens 26 would be doubled in outer diameter as compared to that in the conventional method, which disadvantageously increases difficulties in designing and in production.
Further, if the half mirror 11 is placed between the condenser lens 12 and the optical integrator 5 as shown in FIG. 6, the throughput is extremely lowered with a decrease in power of the exposure light due to a loss in quantity of light. If the reflectance of the half mirror 11 is restricted in order to solve the problem of the light quantity loss, the detection accuracy would be greatly lowered with a decrease of receiving light quantity of the detection system such as the photomultiplier 22. Moreover, a flare could be caused depending upon the arrangement of the half mirror 11, which results in illuminance unevenness on the wafer and measurement errors in reticle alignment.
A so-called annular zone illumination method, in which the reticle is illuminated by a secondary light source formed in annular zone by the optical integrator, may be also employed to illuminate the reticle in the oblique direction in the same manner as in the above plural oblique illumination method, while improving the resolution and the depth of focus of the projection optical system.
In the annular zone illumination method, the reticle is obliquely illuminated in the same manner as in the plural oblique illumination method. The annular zone illumination method could be considered as one of the oblique illumination methods, and, therefore, the annular zone illumination method and the plural oblique illumination method will be hereinafter generally referred to as an oblique illumination method.
However, if the half mirror 11 is placed between the condenser lens 12 and the optical integrator 5 as shown in FIG. 6, there would be problems caused in designing and in production of the condenser lens 26 as well as problems of illuminance unevenness due to the light quantity loss, the flare, and the like and of measurement errors, similarly as described.
Second prior art
A reduction projection exposure system is used to replicate a pattern of a reticle through a projection optical system onto a wafer in production of semiconductor devices or liquid crystal display devices with use of the photolithography technique. One of important properties of such a projection exposure system is the overlay accuracy. A main factor giving an influence to the overlay accuracy is a magnification error of the projection optical system. A line width in a pattern, which is used for very large scale integrated circuit (VLSI) for example, becomes finer and finer with years, and a need to improve the overlay accuracy on the wafer becomes greater accordingly. There is then another need for keeping the projection magnification of the projection optical system within a certain range.
It is known that the projection magnification of the projection optical system fluctuates in the vicinity of a desired magnification because of a slight temperature change in the system, a slight pressure change in atmosphere in a clean room, a temperature change of atmosphere, and irradiation hysteresis of exposure light on the projection optical system. Some of conventional projection exposure systems have a magnification correction mechanism for finely adjusting the magnification of the projection optical system to obtain a desired magnification. Examples of such a magnification correction mechanism are a mechanism for changing a certain lens gap in the projection optical system and a mechanism for adjusting a pressure in an air chamber in the projection optical system.
A position of best imagery plane (focal plane) of the projection optical system and the distortion condition also change due to the same changing factors as those in magnification. There are projection exposure systems provided with a focusing correction mechanism and/or with a distortion correction mechanism as well as with the magnification correction mechanism. For example, the distortion condition of the projection optical system may be adjusted to some extent by slightly inclining the reticle with respect to a plane normal to the optical axis of the projection optical system.
FIG. 12 shows a simplified structure of a conventional projection exposure system provided with a control mechanism of imaging characteristic of the projection optical system. In FIG. 12, reference numeral 101 designates a mercury lamp. Exposure light IL emitted from the mercury lamp 101 is converged by an elliptic mirror 102, and is then reflected by a mirror 103 to enter an input lens 104. The exposure light IL is converted by the input lens 104 into a substantially parallel beam. There is a shutter 106 disposed between the elliptic mirror 102 and the mirror 103. The supply of the exposure light IL to the input lens 104 may be stopped by closing the shutter 106 through a drive motor 107. Numeral 108 denotes a main control system for controlling operations of the entire system, which controls the operation of the drive motor 107. A pulse laser source such as KrF excimer laser or another light source may be employed in place of the mercury lamp 101.
The exposure light IL output from the input lens 104 enters a fly eye lens 105 as an optical integrator, and numerous images are formed as a secondary light source on a focal plane of the fly eye lens 105 on the output side (on the reticle R side). An aperture stop 109 is located at the secondary light source formation plane.
The exposure light IL output from an aperture portion of the aperture stop 109, that is, from the secondary light source, is directed to a beam splitter 110 having a high transmittance but a low reflectance. The exposure light IL 1 reflected by the beam splitter 110 enters a light receiving surface of an integrated exposure amount monitor 111 comprised of a photoelectric converter. A detection signal of the integrated exposure amount monitor 111 is supplied to the main control system 108. The main control system 108 calculates an integrated exposure energy by multiplying the integration value of the detection signal by a certain factor. The exposure light IL having passed through the beam splitter 110 passes through a first relay lens 112, a variable reticle blind 113, a second relay lens 114, and a mirror 115 to enter a main condenser lens 116. The exposure light IL is moderately converged by the main condenser lens 116 to illuminate the reticle R at a substantially uniform illuminance. In this arrangement, the variable reticle blind 113 is located on a plane conjugate with the reticle R and functions as an illumination field stop of the exposure light IL to the reticle R. The main control system 108 controls a drive unit 117 to set a shape of an aperture portion of the variable reticle blind 113 in a certain state.
The exposure light IL having passed through a pattern area on the reticle R is converged on a shot area on a wafer W through the projection optical system PL, whereby a pattern of the reticle R is replicated at a predetermined reduction magnification on the shot area on the wafer W. A Fourier transform plane (pupil plane) of the projection optical system PL is conjugate with the secondary light source formation plane of the fly eye lens 105. Further, the wafer W is held on a Z stage 118, and the Z stage 118 is mounted on an X-Y stage 119. The X-Y stage 119 may two-dimensionally position the wafer W in a plane normal to the optical axis of the projection optical system PL, and the Z stage 118 may position the wafer W in a direction parallel to the optical axis of the projection optical system PL. The main control system 108 controls the operations of the X-Y stage 119 and the Z stage 118 through a control unit 120.
In FIG. 12, numeral 121 denotes an irradiation optical system of a focus detection system, and 122 a light receiving optical system of the focus detection system. The irradiation optical system 121 obliquely projects an image for example of a slit pattern onto an object on the Z stage 118 in an exposure area of the projection optical system PL, and the light receiving optical system 122 receives the image of the slit pattern for example on a vibrating slit plate therein. A photoelectric detector is placed behind the vibrating slit plate. An output signal of the photoelectric detector is subject to synchronous rectification with a drive signal of the vibrating slit plate to obtain a focus detection signal. The thus-obtained focus detection signal is supplied to the main control system 108. The main control system 108 adjusts a height of the Z stage 118 as to keep the focus detection signal at a predetermined level, so that a surface of the object on the Z stage 118 is aligned with the best imagery plane of the projection optical system PL. There is a mechanism for offset adjustment of the focus detection signal, for example, a parallel flat glass an inclination angle of which can be adjusted, provided inside the light receiving optical system 122.
There are an irradiation amount monitor 123 comprised of a photoelectric conversion device, and a reference reflection plate 124 providing a reference for reflectance disposed in the vicinity of the wafer W on the Z stage 118. An irradiation power of the exposure light IL through the projection optical system PL may be measured by moving the X-Y stage 119 to set the irradiation amount monitor 123 in the exposure area of the projection optical system PL. A detection signal of the irradiation amount monitor 123 is supplied through a signal line not shown to the main control system 108.
Numeral 125 represents a characteristic control unit, which adjusts a certain pressure in an air chamber inside the projection optical system PL or which adjusts a certain gap between lenses forming the projection optical system PL. The imaging characteristic such as the projection magnification of the projection optical system PL may be adjusted within a certain range by adjusting the pressure in the air chamber or by adjusting the gap between lenses. The characteristic control unit 125 may further comprise a mechanism for slightly inclining the reticle R to a plane normal to the optical axis of the projection optical system PL. The main control system 108 sets the imaging characteristic such as the projection magnification of the projection optical system PL in a certain state through the characteristic control unit 125, depending upon integrated exposure amount of the exposure light and the reflectance of the wafer W.
The characteristic control unit 125 may control an operation of an offset adjustment mechanism of the focus detection signal in the light receiving optical system 122 of the focus detection system. When it is presumed that the position of the best imagery plane of the projection optical system PL is changed, the main control system 108 adjusts the offset of the focus detection signal of the light receiving optical system 122 through the characteristic control unit 125. By this, the exposure surface on the wafer W is always set on the best imagery plane of the projection optical system PL.
Further, though omitted in FIG. 12, there is a measuring apparatus is provided for measuring a pressure and a temperature around the projection optical system PL. Information about the pressure and the temperature obtained by the measuring apparatus is supplied to the main control system 108.
The exposure light reflected by the wafer W passes through the projection optical system PL, the reticle R, the main condenser lens 116, the mirror 115, the second relay lens 114, the variable reticle blind 113, and the first relay lens 112 to return to the beam splitter 110. The exposure light IL 2 reflected by the beam splitter 110 enters the light receiving surface of the reflectance monitor 126 composed of the photoelectric conversion device. A detection signal detected by the reflectance monitor 126 is supplied to the main control system 108. The main control system 108 moves the X-Y stage 119 through the drive unit 120 to sequentially set the wafer W and the reference reflection plate 124 in the exposure area of the projection optical system PL, whereby detection signals of the reflectance monitor are obtained corresponding to the wafer W and the reference reflection plate 124. The reflectance of the wafer W may be obtained for example from a proportional relation by preliminarily storing the reflectance of the reference reflection plate 124. An example of the method for measuring the reflectance of the wafer W by means of the reflectance monitor 126 is disclosed in U.S. Pat. No. 4,780,747.
In the projection exposure system of FIG. 12, an ordinary secondary light source, light of which is distributed on a circle in a region including the optical axis of the illumination optical system, is used as the secondary light source of the exposure light. The .sigma. value representing a degree of coherency of the exposure light IL, which is a ratio of a radius of an image of the secondary light source on the pupil plane of the projection optical system to a radius of the aperture stop of the projection optical system PL, is fixed for example at about 0.5, because the line width of pattern on the reticle R to be replicated ha been comparatively wide heretofore. In this arrangement, a beam of the exposure light IL is comparatively fine immediately after the secondary light source defined by the aperture stop 109, and, therefore, the beam splitter 110 employed may be of a relatively small size, whereby a less load is forced on the structure of the illumination optical system.
Problems in the second prior art
However, as the pattern of the reticle R to be replicated becomes finer and finer these days, some exposure systems employ an illumination optical system having a larger .sigma. value than 0.5. There has also been developed a projection exposure system which can use a phase shift reticle and a normal reticle as the reticle R while switching therebetween. An illumination optical system in such an exposure system requires a mechanism for changing the .sigma. value in a comparatively wide range. Applicants have proposed the so-called plural oblique illumination method in which the resolution of the projection optical system can be increased while maintaining the depth of focus comparatively deep. In the plural oblique illumination, the secondary light source of the exposure light is formed for example by four modified secondary light sources eccentric to the optical axis of the illumination optical system and separate from each other, and the reticle is obliquely illuminated by the four secondary light sources for example in four directions.
In the plural oblique illumination method, since the four secondary light sources of the exposure light are distributed in a region apart from the optical axis of the illumination optical system, a circle circumscribing the entire secondary light sources has a large radius, whereby the substantial c value of the illumination optical system becomes approximately 1. With the .sigma. value of about 1, an outer diameter of the secondary light sources, corresponding to the opening of the aperture stop 109 in FIG. 12, increases, whereby increasing a diameter of beams of the exposure light output from the secondary light sources.
Further, as the pattern to be replicated becomes finer and finer, the requirements accuracy of illuminance uniformity of the exposure light illuminating the reticle R is required to be higher and higher and the requirements accuracy of illumination telecentricity, which is a degree of parallel of the exposure light, is also required to be higher. Under such circumstances, if a beam splitter for splitting the exposure light is placed between the optical integrator such as the fly eye lens and the lens system such as the relay lens, accuracies for other elements in the illumination optical system must be set extremely high enough to obtain a required imaging characteristic, or the number of necessary lenses is increased, whereby a hard load is forced on the designing and the production of the illumination optical system.